The 404.php file is called when there is a ‘page not found’ condition.

404.php

This php file calls and displays the header, then a message indicating that the ‘page is not found’ is displayed, followed by a search form, to allow the user to do a further search for what they were looking for, then the php file calls for and displays the footer. Notice that, in the Starkers theme anyway, the sidebar is not displayed on the 404 page.